The App Engine SDK includes a web server application you can use to
test your application. The server simulates the App Engine environment
and services, including sandbox restrictions, the datastore, and the
services.

You can start the development server within the Eclipse debugger. In the Run menu, select Debug As > Web Application. See Using the Google Plugin for Eclipse for details on creating the debug configuration.

Slim3 supports HOT reloading, so you can continue to develop this tutorial without restarting your application.